Partially it is a scale thing. The state only has so many inspectors, so if you limit the law to large industrial users you don't need to have inspectors driving around to tens of thousands of tiny single-use boilers inspecting them for environmental requirements they would never be able to afford in the first place.
Then it seems like scale should be part qualifying for the exemption. In this case the power plant is absolutely at a large enough scale that it should be regulated.
